Method for the recovery of caprolactam

ABSTRACT

A process comprising the steps of: A. CONTACTING AT A TEMPERATURE RANGING FROM ABOUT 40* C. to about 90* C. a rearrangement mass comprising Epsilon caprolactam and sulfuric acid with the following reactants: (I) SUBSTANTIALLY WATER-FREE AMMONIUM SULFATE IN AN AMOUNT RANGING FROM ABOUT 0.9 TO ABOUT 1.1 MOLES PER MOLE OF SULFURIC ACID PRESENT IN SAID REARRANGEMENT MASS, AND (II) AN AQUEOUS AMMONIUM BISULFATE SOLUTION SATURATED AT A TEMPERATURE WITHIN THE RANGE OF 20* TO 55* C., the amount of said aqueous solution being such as to dissolve at the contacting temperature the ammonium bisulfate formed by the reaction of the sulfuric acid component of the rearrangement mass with said ammonium sulfate, to thereby afford an Epsilon -caprolactam phase and an aqueous ammonium bisulfate phase; B. SEPARATING SAID Epsilon -CAPROLACTAM PHASE; C. COOLING SAID AQUEOUS AMMONIUM BISULFATE PHASE TO A TEMPERATURE WITHIN THE RANGE OF 20* TO 55* C. to thereby afford crystalline ammonium bisulfate and a saturated aqueous ammonium bisulfate solution; D. RECOVERING AND RECYCLING SAID SATURATED AQUEOUS SOLUTION TO STEP A).

United States Patent Beckham [72] Inventor: Leland J. Beckham, Lutz,Fla.

[73] Assignee: Allied Chemical Corporation, New

' York, N.Y.

[22] Filed: Aug. 27, 1971 [21] Appl. No.: 175,740

[52] US. Cl. ..260/239.3 A [51] Int. Cl. ..C07d 41/06 [58] Field ofSearch ..260/239.3 A

[56] References Cited UNITED STATES PATENTS 3,264,060 8/1966 Nieswandtet al. .260/2393 A 3,336,298 8/1967 De Rooij ..260/239.3 A

METHOD FOR THE RECOVERY OF CAPROLACTAM ABSTRACT A process comprising thesteps of:

[ 51 3,694,433 51 Sept. 26,1972

a. contacting at a temperature ranging from about 40 C. to about 90 C. arearrangement mass comprising e-caprolactam and sulfuric acid with thefollowing reactants:

(i) substantially water-free ammonium sulfate in an amount ranging fromabout 0.9 to about 1.1 moles per mole of sulfuric acid present in saidrearrangement mass, and

(ii) an aqueous ammonium bisulfate solution saturated at a temperaturewithin the range of 20 to 55 C., the amount of said aqueous solutionbeing such as to dissolve at the contacting temperature the ammoniumbisulfate formed by the reaction of the sulfuric acid component of therearrangement mass with said ammonium sulfate, to thereby afford ane-caprolactam phase and an aqueous ammonium bisulfate phase;

b. separating said e-caprolactam phase; c. cooling said aqueous ammoniumbisulfate phase d. recovering and recycling said saturated aqueoussolution to step a).

5 Claims, No Drawings BACKGROUND OF THE INVENTION The present inventionrelates to the recovery of caprolactam from a Beckmann rearrangementmass.

It is well known that lactams can be obtained by the Beckmannrearrangement of alicyclic oximes, the rearrangement normally beingeffected in a concentrated acid medium which is usually sulfuric acid oroleum. ecaprolactam is conventionally produced by reacting cyclohexanonewith hydroxylamine to afford cyclohexanone oxime and then rearrangingthis oxime by contacting with oleum to afford e-caprolactam. In thisprocedure, the lactam formed is in the conjugated acid form admixed withsulfuric acid in a viscous, intractable, one-phase system generallyreferred to in the art as rearrangement mass." To effect separation ofthe ecaprolactam, the rearrangement mass is'usually contacted withsufficient aqueous ammonium hydroxide to neutralize the sulfuric acidcomponent of the rearrangement mass, thereby affording aqueous ammoniumsulfate solution as one phase and e-caprolactam as a second phase.Ammonium sulfate, although agronomically satisfactory as a fertilizer,is losing its share of this market to more concentrated sources ofnitrogen, such as urea, which are less costly to ship. Production oflarge quantities of ammonium sulfate is therefore not desirable and canresult in the accumulation of vast quantities of essentially unsaleablematerial which poses a problem of storage and, frequently, a pollutionproblem.

It has more recently been proposed (U.S. Pat. No. 3,336,298) to freee-caprolactam from the rearrangement mass by adding thereto aqueousammonium sulfate wherein the mole ratio of ammonium sulfate to sulfuricacid'is at least 0.75: l. The ammonium sulfate serves to convertthelact'am from the conjugated acid form to the separated free baseform. The sulfuric acid is simultaneously converted to ammoniumbisulfate which'retains the acid values thereof and can therefore beused for other purposes, e.g. in the extraction of phosphoric acid fromphosphate rock. Altemately, the ammonium bisulfate can be pyrolyzed inaccordance with the teaching of US. Pat. No. 3,282,646 -or 3,383,170 toafford sulfur dioxide and ammonia. This procedure suffers from thedisadvantage, however, that recovery of the desired lactam is dependenton the ammonium sulfate to sulfuric acid ratio employed, with trulysatisfactory yields of separated lactam being obtained only at ratherhigh salt to acid ratios. For example, the optimum yield ofe-caprolactam with this procedure occurs at a molar ratio within therange of 1.6 to 2.0 moles of ammonium sulfate per mole of sulfuric acid.This means that after reaction, from about 0.6 to 1.0 moles of excessunreacted ammonium sulfate will be present in aqueous solution for eachmole of ammonium bisulfate. The presence of unreacted sulfate isundesirable Since it is wasteful, uneconomical and entails extraseparatory and/or purification steps to 2 SUMMARY OF THE INVENTION Myprocess entails the addition to the rearrangement mass of:

a. ammonium sulfate in solid form which is trans formed into thecorresponding acid salt (ammonium bisulfate) on reaction with thesulfuric acid component of the rearrangement mass; and

b. an aqueous solution of ammonium bisulfate, the corresponding acidsalt of ammonium sulfate. The quantities of solid salt and acid saltsolution and the temperature of reaction thereof with the rearrangementmass are all critical parameters of the instant process. High yields ofrecovered lactam are realized when employing a combination of controlledtemperature and essentially equimolar amounts of ammonium sulfaterelative to the sulfuric acid content of the rearrangement mass, therebyavoiding having large amounts of unreacted ammonium sulfate left insolution.

The water and ammonium bisulfate are recycled and hence the only netingredient added is ammonium sulfate in an amount equimolar to thesulfuric acid present in the rearrangement mass. I have additionallyfound that chloroform has an unexpectedly high capacity for dissolvinglactam when employed as an extraction solvent resulting in increasedrecovery of the lactam after its separation from the rearrangement mass.

DETAILED DESCRIPTION OF THE INVENTION In the practice of the presentprocess, an alicyclic oxime, preferably cyclohexanone oxime, isrearranged in known manner with oleum to afford the correspondinglactam. The resulting rearrangement mass, i.e. lactam in the conjugatedacid form together with sulfuric acid, is then separated into atwo-phase system by the addition to the rearrangement mass of an aqueoussolution of ammonium bisulfate and, as a dry solid, ammonium sulfate inan amount sufficient to provide an equimolar ratio of ammonium sulfateto sulfuric acid. The dry ammonium sulphate and aqueous ammoniumbisulfate can be added to the rearrangement mass either simultaneouslywhich is preferable, or either one first. The reaction of rearrangementmass with ammonium sulfate and ammonium bisulfate is effected at atemperature above about 40 C. but not greater than about C. Said lactamis thereby transformed into the free base form as a separate phase andis then recovered, preferably employing a water immiscible organicsolvent which has a high capacity for dissolving said lactam, whichsolvent can also be added either prior to, simultaneously with orsubsequent to the ammonium bisulfate and ammonium sulfate. Ammoniumsulfate fulfills the solubility requirements of the instant processwater-immiscible organic solvent for the lactam would also be present,as heretofore indicated.

The parameters of temperature and the molar proportions of the reactantsemployed herein in separating the lactam from the rearrangement mass areessential to the successful operation of the instant invention.Operation of the instant process at temperatures above about 90 C. willresult in the loss of some of the lactam through hydrolysis thereof toaminocaproic acid. The preferred reaction temperature will be about 60to 80 C. Molar ratios of ammonium sulfate to sulfuric acid in excess of1.1 to l are not employed in the instant process since they result inthe presence of unreacted ammonium sulfate and offer no advantage inproduct yield under the instant process conditions. Conversely, ratiosbelow 0.9 to 1 will result in a reduced recovery of lactam under thetemperature limitations employed herein. Preferably, the mole ratio ofammonium sulfate to sulfuric acid will be as close to 1:1 as canreasonably be obtained in an industrial type process.

As hereinafter employed, the term about 1:1 as applied to the ammoniumsulfate to sulfuric acid mole ratio shall connote a ratio ranging fromabout 0.9 to l to 1.1 to 1.

In the process of the instant invention, solid ammonium sulfate and anaqueous solution of ammonium bisulfate are added to the rearrangementmass in a first reaction zone, preferably in conjunction with a lactamextraction solvent. The ammonium sulfate in the presence of sulfuricacid and water reacts immediately to produce more ammonium bisulfate,which results in the formation of an amount sufficient to produce asaturated solution thereof at a temperature within the prescribed rangeof 40-90 C.; for example, 60 C. This temperature, herein referred to asT is that temperature at which the aqueous solution of ammoniumbisulfate present with rearrangement mass becomes saturated upon theaddition of the amount of ammonium sulfate necessary to provide anammonium sulfate to sulfuric acid ratio of about 1:1. At this point, theseparate phase containing lactam is removed from the aqueous bisulfatephase. The saturated aqueous solution of ammonium bisulfate is thencooled to a lower temperature within the range of 55 C.; for example, 40C., ordinarily in a second reaction zone, and this lower temperature, Tpermits crystallization of a portion of the ammonium bisulfate due to adecrease in the solubility thereof at the lower temperature. Theammonium bisulfate which crystallizes out is removed and the resultingammonium bisulfate aqueous solution, which is now saturated at T butwould be unsaturated at T is recycled to the first reaction zonemaintained at T1 for reuse in a subsequent lactam separation procedureby addition of more rearrangement mass and ammonium sulfate, which ofcourse reacts immediately with the sulfuric acid to form additionalammonium bisulfate. Although the operable ranges for T and T overlap, inany particular run T, must, of course, be higher than T The process ofthe instant invention is thus seen to comprise:

a. contacting, at a temperature in the range of 40 C.

to 90 C., rearrangement mass comprising e-caprolactam and sulfuric acidwith i. solid ammonium sulfate in an amount ranging from about 0.9 toabout 1.1 moles per mole of sulfuric acid present in the rearrangementmass, and

an aqueous ammonium bisulfate solution which is saturated at atemperature within the range 20 to 55 C., the amount of said aqueoussolution being sufficient to dissolve the additional ammonium bisulfateformed by the reaction of the sulfuric acid component of therearrangement mass with said ammonium sulfate at a temperature of 40 toC. to thereby afford an caprolactam phase and an aqueous ammoniumbisulfate phase essentially saturated at 5090 C b. removing saide-caprolactam phase.

0. cooling said aqueous ammonium bisulfate phase to a lower temperaturewithin the range of 2055 C.

d. recovering the ammonium bisulfate which crystallizes out on saidcooling to thereby afford an essentially saturated at 2055 C. aqueoussolution of ammonium bisulfate, and

e. recycling said saturated aqueous solution to step This procedure isparticularly advantageous and efficient since it avoids the costs ofwater evaporation and permits continuous reuse of the same ammoniumbisulfate solution in a cyclic process. Thus, once the reaction has beenstarted, only rearrangement mass and sufficient ammonium sulfate toneutralize the sulfuric acid in the rearrangement mass, and provide asaturated solution of ammonium bisulfate at T need be added to thesystem, and sufficient water to make up any water removed in thee-caprolactam phase. Recovery of the desired lactam is realized in highyield.

The comparative solubility of ammonium bisulfate at the temperatures inquestion is illustrated by the following table.

ABS ammonium bisulfate Recovery of the lactam after separation from therearrangement mass is preferably effected, as above indicated, byextraction with one of the many known water-immiscible organic solventswhich dissolves ecaprolactam. The solvent is preferably added to thereaction vessel along with the rearrangement mass, ammonium sulfate andaqueous ammonium bisulfate and the resultant phase containing lactam andorganic solvent removed from the aqueous ammonium bisulfate phase. Ifdesired the ammonium bisulfate phase can be further extracted withadditional solvent in a single or multi-stage process to remove alltrace of e-caprolactam therefrom. Suitable solvents include benzene,

toluene, xylene, dichloroethane, trichloroethane, ethyl ether, dibutylcarbitol, chloroform, and the like. When a solvent is utilized anadditional advantage is that no water is extracted into thee-caprolactam phase and hence none need be added to the system as makeup.

It has been discovered that chloroformhas an unexpectedly high capacityto dissolve the lactam when employed as solvent and results inessentially quantitative recovery of lactam. This is particularlysurprising since such extraordinary capacity to dissolve lactam is notshared by closely related solvents such as dichloroandtrichloroethylene. Chloroform is therefore an especially preferredsolvent herein.

The invention is further illustrated by the examples which follow.Preparation of Rearrangement Mass Approximately Kg. of rearrangementmasswas obtained by reacting cyclohexanone oximewith oleum containing 3%80;. The rearrangement was effected by adding the oxime incrementally tothe total required amount of oleum while maintaining the'temperature atabout 80 C.

' Aliquots of this rearrangement mass were utilized in the recoveriesoflactam reported in the Examples which follow.

EXAMPLE 1 A reaction vessel was equipped with a stirrer and a coolingbath. 13.45 parts of the rearrangement mass containing 7.18 partssulfuric acid as prepared above were addedto 75 parts chloroform solventafter which 96.6 parts of an aqueous solution of ammonium bisulfate wereadded thereto. 9.6 parts of solid ammonium bisulfate was then added tothe contents of the vessel which was cooled sothat the temperature ofits contents was maintained atabout 60 C. There thereby results aseparation of e-caprolactam' in chloroform as an upper phaseand asaturated solution of ammonium bisulfate as a lower phase. The phaseswere mixed for about three minutes after which the upper lactam/solventphase was decanted and the aqueous phase then extracted twice with 75partseach time of chloroform. The chloroform extracts were combined withthe ecaprolactam phase in a suitable vessel and the solution stripped toremove the chloroform. Recovery of the lactam was essentiallyquantitative.

The aqueous lower phase, a saturated solution of ammonium bisulfate at60 (3., was removed from the reaction vessel to a cooler maintained at atemperature of about 40 C. whereby 16.8 parts of ammonium bisulfatecrystallized out, the supernatant solution, 96.6 parts of aqueousammonium bisulfate, was returned to the reaction vessel and theforegoing process repeated using additional ammonium sulfate, chloroformand rearrangement mass.

EXAMPLE 2 The process of Example 1 was repeated except that.

only 50 parts of chloroform were initially added. After de'cantation ofthe chloroforml-caprolactarn phase, the aqueous ammonium bisulfate phasewas further extracted in a 4-stage counter-current extraction apparatusemployed a 3/1 weight ratio of chloroform to 6 bisulfate phase in eachstage. All extracts were combined and the chloroform solvent strippedoff affording an essentially quantitative yield of e-caprolactam.

EXAMPLE 3 The procedure of Example 2 was followed except that in eachrun an equivalent weight of the indicated solvent was substituted forchloroform. Yields of recovered e-caprolactam are tabulated below:

13.45 parts of rearrangement mass containing 7.18 parts H was stirred at60 C. together with 9.7 part solid ammonium sulfate and 96 parts of anammonium bisulfate solution which is saturated with respect to bothammonium bisulfateand caprolactam at 40 C. Such a solution will containabout 79.7 percent by weight ammonium bisulfate. After agitation for 3minutes, the mixture was allowed to settle forming two layers. The upperlayer, amounting to 8.9 parts contains the caprolactam component of therearrangement mass. The upper layer was decanted and distilled to affordpure caprolactam. The lower layer was cooled to 40 C.precipitatingammonium bisulfate crystals, amounting to 16.9 parts. Themother liquor at 40 C. was then recycled to react with morerearrangement mass. A small amount of water, i.e. about 2.3 parts wasadded to compensate for water removed with the crude lactam.

I claim:

l. A process comprising the steps of:

a. contacting at a temperature ranging from about 40 C. to about C. arearrangement mass com prising e-caprolactam' and sulfuric acid with thefollowing reactants:

i. substantially water-free ammonium sulfate in an amount ranging fromabout 0.9 to about 1.1

moles per mole of sulfuric acid present in said rearrangement mass, and

ii. an aqueous ammonium bisulfate solution saturated at a temperaturewithin the range of 20 to 55 C., the amount of said aqueous solutionbeing such as to'dissolve at a temperature within the range of 40 to 90C. the ammonium bisulfate formed by the reaction of the sulfuric acidcomponent of the rearrangement mass with said ammonium sulfate, tothereby afford an ecaprolactam phase and an aqueous ammonium bisulfatephase;

b. separating said e-caprolactam phase;

c. cooling said aqueous ammonium bisulfate phase to a lower temperaturewithin the range of 20 to 55 C. to thereby afford crystalline ammoniumbisulfate and a saturated aqueous ammonium bisulfate solution;

d. recovering and recycling said saturated aqueous 7 solution to stepa).

2. A process in accordance with claim 1 wherein said rearrangement massis contacted with an additional reactant which is a water-immiscibleorganic solvent for e-caprolactam.

solvent for e-caprolactam.

5. A process in accordance with claim 1 wherein said contactingtemperature ranges from about 60 C. to about C.

2. A process in accordance with claim 1 wherein said rearrangement massis contacted with an additional reactant which is a water-immiscibleorganic solvent for epsilon -caprolactam.
 3. A process in accordancewith claim 2 wherein said solvent is chloroform.
 4. A process inaccordance with claim 2 wherein said aqueous ammonium bisulfate phaseafter separation of the epsilon -caprolactam phase and prior to saidcooling is extracted at least once with a water-immiscible organicsolvent for epsilon -caprolactam.
 5. A process in accordance with claim1 wherein said contacting temperature ranges from about 60* C. to about80* C.